Methodic doubt is a systematic process of withholding assent regarding the truth or falsehood of all one’s beliefs until they have been demonstrated or rationally proven to be true or false. This method was introduced into the field of philosophy at the advent of the modern period. With the rise of the Scientific Revolution, some philosophers thought that by imitating the methodical nature of the natural sciences with its scientific method, philosophy too could demonstrate certain, indubitable truths. This attempt was made by the famous French philosopher and mathematician René Descartes (1596-1650), who is often referred to as the “Father of Modern Philosophy.” Through the use of methodic doubt Descartes attempted to demonstrate philosophical truths, which he thought could defeat the most radical doubt or skepticism. Such a method or way of doing philosophy has become a hallmark of modern philosophy, particularly within the rationalist tradition.
Descartes exemplified his methodic doubt in "Meditation 1" of his classic Meditations on First Philosophy. Descartes began by stating that throughout his life he had acquired many opinions and beliefs which he later discovered to be false. Also, he had read many books and found that the most profound thinkers disagreed on the most important issues. But what Descartes sought was a truth beyond dispute, a certainty that could not be doubted. The problem was how to achieve this aim. Clearly he could not filter through each and every idea he had acquired over his entire lifetime. So he devised a systematic method by which he could set aside huge numbers of ideas. First, he determined that any belief or supposed knowledge that admitted even the slightest doubt could not be held as true. This did not mean he had to disbelieve it, but simply suspend his judgment regarding it. Secondly, Descartes determined that different kinds of opinions could be grouped according to the principle or foundation upon which they were based. So if the underlying principle could be doubted, then all beliefs based on that principle could be set aside. In this way, Descartes set out to “demolish the house” of all his former opinions in order to rebuild the foundation upon which all true knowledge was to be built. The methodic doubt that he employed was carried out in three systematic steps.
The first step of Descartes’s methodic doubt was to question all knowledge that he had acquired through the senses. He determined that if the senses had deceived him even once, they were no longer trustworthy. But in the past his senses had deceived him. For example, he often mistook what he saw at a distance (like a tree that appeared to be a man). Moreover, even the impressions he received in the moment, when he was directly looking at something (like the fire before him when he was writing), proved not to be reliable. For how did he know that he was not really dreaming? Descartes tried to answer this question by arguing that our immediate impressions are so vivid they must be real; immediately he countered this argument, however, by recalling that he often dreamed of sitting before the fire while writing, and that some of these dreams appeared to him quite vividly. Thus, he could not be sure that he was not dreaming at that very moment. In any case, from this doubt Descartes determined that all the knowledge received through the senses must be suspended.
The second step of Descartes’s methodic doubt began by accepting for argument’s sake that everything may very well be a dream. But if so, then there is still some knowledge to be had from within the dream itself. In other words, although we can doubt that all images and ideas that we experience in our dreams refer to some outward reality, we nevertheless can consider them in their simple components to see if any of them might be true in themselves. After dismissing all simple components which are based on senses, such as color, sound, etc., Descartes arrived at the simple truths of mathematics. The truth, for example, that 2 + 2 = 4 does not rely on any sensible experience but is grasped entirely in our minds regardless of whether we are dreaming or awake. It would seem, then, that Descartes has reached a clear and distinct idea, one which is beyond all doubt. For we can say outwardly in words that 2 + 2 = 5, but we cannot really think in our minds that it equals anything but 4.
The final step in Descartes’s methodic doubt is what is often referred to as “hyperbolic doubt.” Recall that Descartes is searching for a truth that is beyond any doubt, even the slightest. For this reason, if one can offer a possible explanation, even if it is implausible or far-fetched, then it casts doubt upon the belief and makes it uncertain. So how can we doubt that 2 + 2 does not really equal 4, if whenever we think 2 + 2, our minds must necessarily admit that it equals 4? Here Descartes introduced what he called the “evil genius” or “malicious deceiver.” He hypothesized that perhaps there is a malicious god who enjoys deceiving us. Although we always think 2 + 2 = 4, perhaps this god is really tricking us and in reality it equals 5. Descartes likens this to situations where we are “absolutely” confident regarding our belief about a certain fact or state-of-affairs; and yet despite this confidence we often discover later that we were mistaken. The same might hold for our mathematical truths in which some evil god is deceiving us.
These three steps completed Descartes’s methodic doubt in which he attempted to clear his mind of all previous beliefs in order to found truths that are beyond doubt. The remainder of the Meditations was devoted to this task and by the end of it Descartes claims to have demonstrated beyond doubt the certainty of both the human mind (or Cogito) and the existence of God. These, then, were the foundational, philosophical truths that all our other knowledge can be built upon. Other modern philosophers who follow Descartes, though not using the same steps as his, often employ a methodic doubt as well in their search for a philosophical foundation for all truth and knowledge.
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